OP191 datasheet(20/24 Pages) AD

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OP191/OP291/OP491

Rev. E | Page 20 of 24

A 2.5 V REFERENCE FROM A 3 V SUPPLY

In many single-supply applications, the need for a 2.5 V

reference often arises. Many commercially available monolithic

2.5 V references require a minimum operating supply voltage of

4 V. The problem is exacerbated when the minimum operating

system supply voltage is 3 V. The circuit illustrated in Figure 67

is an example of a 2.5 V reference that operates from a single

3 V supply. The circuit takes advantage of the OP291 rail-to-rail

input and output voltage ranges to amplify an AD589 1.235 V

output to 2.5 V. The OP291 low TCVOS of 1 μV/°C helps

maintain an output voltage temperature coefficient of less than

200 ppm/°C. The circuit overall temperature coefficient is

dominated by the temperature coefficient of R2 and R3. Lower

temperature coefficient resistors are recommended. The entire

circuit draws less than 420 μA from a 3 V supply at 25°C.

RESISTORS = 1%, 100ppm/°C

POTENTIOMETER = 10 TURN, 100ppm/°C

100kΩ

1/2

OP291

100kΩ

5kΩ

2.5V

REF

17.4kΩ

AD589

Figure 67. A 2.5 V Reference that Operates on a Single 3 V Supply

5 V ONLY, 12-BIT DAC SWINGS RAIL-TO-RAIL

The OPx91 family is ideal for use with a CMOS DAC to

generate a digitally controlled voltage with a wide output range.

Figure 68 shows the DAC8043 used in conjunction with the

AD589 to generate a voltage output from 0 V to 1.23 V. The

DAC is operated in voltage switching mode, where the reference

is connected to the current output, IOUT, and the output voltage

is taken from the VREF pin. This topology is inherently

noninverting as opposed to the classic current output mode,

which is inverting and, therefore, unsuitable for single supply.

17.8kΩ

AD589

232Ω

32.4kΩ

100kΩ

VOUT = –––– (5V)

4096

GND CLK SR1

DIGITAL

CONTROL

VREF

RFB

VDD

IOUT

1.23V

DAC8043

1/2

OP291

Figure 68. 5 V Only, 12-Bit DAC Swings Rail-to-Rail

The OP291 serves two functions. First, it is required to buffer

the high output impedance of the DAC VREF pin, which is on the

order of 10 kΩ. The op amp provides a low impedance output

to drive any following circuitry. Second, the op amp amplifies

the output signal to provide a rail-to-rail output swing. In this

particular case, the gain is set to 4.1 to generate a 5.0 V output

when the DAC is at full scale. If other output voltage ranges are

needed, such as 0 V to 4.095 V, the gain can easily be adjusted

by altering the value of the resistors.

A HIGH-SIDE CURRENT MONITOR

In the design of power supply control circuits, a great deal of

design effort is focused on ensuring a pass transistor’s long-

term reliability over a wide range of load current conditions.

As a result, monitoring and limiting device power dissipation

is of prime importance in these designs. The circuit illustrated

in Figure 69 is an example of a 5 V, single-supply, high-side

current monitor that can be incorporated into the design of a

voltage regulator with fold-back current limiting or a high

current power supply with crowbar protection. This design uses

an OP291 rail-to-rail input voltage range to sense the voltage

drop across a 0.1 Ω current shunt. A p-channel MOSFET used

as the feedback element in the circuit converts the op amp

differential input voltage into a current. This current is then

applied to R2 to generate a voltage that is a linear representation

of the load current. The transfer equation for the current

monitor is given by

SENSE

Output

Monitor

⎟

⎠

⎞

⎜

⎝

⎛

For the element values shown, the monitor output transfer

characteristic is 2.5 V/A.

RSENSE

0.1Ω

3N163

2.49kΩ

MONITOR

OUTPUT

100Ω

1/2

OP291

Figure 69. A High-Side Load Current Monitor

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